A printhead is a part of a computer printer that contains the printing elements. The printing elements include light emitting elements such as lasers that are used to write information such as graphic images and alphabetic text to a drum coated with a light sensitive material such as a selenium compound. The drum acquires a charge proportional to the intensity of the light. The charges on the drum replicate a desired image. The drum is then rotated through a toner application system, which coats the drum with the toner. The thickness of the coat of the toner is controlled by the charge on the drum. The drum continues to rotate and transfers the toner to a blank sheet of paper.
Alternatively, the light emitting elements can be used to directly write an image to a light sensitive medium such as photographic paper. FIG. 1 illustrates how the printhead 10 is used to write a two-dimensional image. The drum 20 or paper 20 moves with respect to the printhead 10, which is held stationary as the paper 20 or drum 20 moves past the printhead 10. Data is fed to the printhead 10 for each line of the image. The size of the image dots written to the drum 20 or paper 20 depends on the velocity of the drum 20 or paper 20. For example, if the printhead 10 holds the line data for one millisecond and the paper 20 moves at the velocity of 10 cm/second the image dot is 0.1 millimeters long.
After the first line is written, the data in the printhead 10 is replaced by the image data for the second line. Since this takes some time, the paper 20 has moved causing a separation from the first image line on the drum 20 or paper 20. The second line is written to the drum 20 or paper 20 when the next line of data is sent to the printhead 10. This process continues until the completed image has been written to the drum 20 or paper 20.
A new organic light emitting diodes (OLED) technology, which replaces the laser with an OLED as the light emitting elements, is simpler, faster and superior in resolution to the laser technology. However, the lack of manufacturing uniformity and differential color aging of the OLED over the lifetime of the products that implement the OLED are hindering the commercialization of the OLED technology.
Nuelight Corporation, the assignee of the present application, has several pending provisional and non-provisional patent applications that relate to improving the use of light emitting elements, for example, OLED, to illuminate displays such as the LCD displays. See, for example, U.S. patent application Ser. No. 10/872,344 entitled Method and Apparatus for Controlling an Active Matrix Display and U.S. patent application Ser. No. 10/872,268 entitled Controlled Passive Display Apparatus and Method for Controlling and Making a Passive Display. Those patent applications relate to the use of feedback systems to control the emissions of the display pixels.
The techniques of the present invention relate to improving the use of light emitting elements, for example, OLED, in printhead applications. The light emitting elements serve different purposes in the printheads than in the displays. In the displays, for example, in the liquid crystal displays (LCD), millions of light emitting elements are arranged in two-dimensional arrays to illuminate the display pixels. In printheads, on the other hand, the light emitting elements are arranged in a linear array to write information to a drum or a photographic paper via emissive pixels.
The challenges associated with the application of the light emitting elements to the displays and the printheads are different. The displays are inherently restrictive in the amount of area the feedback sensor circuitry can occupy because each pixel is surrounded by other pixels, and therefore, a feedback sensor must be included inside a pixel area. The printheads, on the other hand, use linear arrays in which a pixel is not surrounded by other pixels and so the feedback sensor can be mounted outside the pixel, for example, above or below the pixel. The techniques of the present invention relate to using the emission of light emitting elements of a printhead as feedback signals to control the light emitting elements.